The invention relates to a multi-gradation image recording apparatus to be applied to such recording apparatuses as printers, copying machines, and facsimile machines, and more particularly to a recording apparatus capable of producing high-quality multi-gradation images.
Among a variety of multi-gradation image recording apparatuses, a thermosensitive recording apparatus and a thermal transfer recording apparatus, being relatively simple in their structure, are extensively applied to various recording systems such as printers, copying machines, and facsimile machines.
To record multi-gradation images, a thermal transfer recording method using, e.g., sublimated ink sheets is employed. In this method, an amount of color ink, which corresponds to an amount of heat generated by electric energy applied to a plurality of heating resistors constituting a recording thermal head, is transferred onto a recording sheet to record the images. The amount of heat generated by the heating resistors is controlled by the number and duration of electric pulses applied to these resistors.
This thermal transfer recording method allows comparatively satisfactory multi-gradation recording to be achieved with a simple control.
Such a conventional multi-gradation recording system is disclosed in, e.g., Japanese Patent Unexamined Publication No. Sho-60-9271/(1985). FIG. 15 is a chart showing waveforms of conducting pulse SB to be applied to the respective heating resistors constituting the thermal head in this conventional multi-gradation recording system, where t.sub.w is the pulse duration of the conducting pulse SB, t.sub.p is the repetitive cycle of the conducting pulse SB, and N is the number of conducting pulses SB (3 in this example). The number of conducting pulses SB is selected and set in advance on a density basis, each density being expressed in a gradation level. By applying such number of conducting pulses SB as specified every gradation level, a portion of ink commensurate with the energy corresponding to the number of pulses is sublimated, thereby causing an image to be recorded at respective gradation levels. In the recording, usually, the conducting pulses are applied to the corresponding heating resistors arranged in line on the thermal head either collectively or in division. The recording of a plurality of lines is performed while forwarding a recording sheet in an auxiliary scanning direction at a constant speed sequentially.
The multi-gradation recording is performed as described above. Since a major factor defining the recording density of each gradation level is the temperature of each heating resistor disposed on the thermal head, variations in resistance of the heating resistors and variations in temperature due to change in ambient temperature or the like greatly affect the recording density, and this makes it difficult to implement high-quality recording. To overcome such a shortcoming, various correction means have been proposed.
To deal with changes in ambient temperature, a thermistor (temperature-sensitive element) is employed. The thermistor mounted on the thermal head detects changes in temperature so that the duration or number of pulses specified every gradation level are controlled. Therefore, the variations in the recording density of the same gradation level can be suppressed.
To correct density nonuniformity due to variations in resistance of the heating resistors, a measure has been reported in a thesis entitled "Development of a High-Definition Video Copy" (No. 1276 of Preliminary Publications of the Convention of the 1986 Electronic Information Communications Society). Specifically, FIG. 16 is a block diagram showing a means for correcting erratic resistances. In FIG. 16, reference numeral 101 designates a counter for counting a clock; 102, an EPROM (erasable programmable read only memory) which receives count data from the counter 101 as an address and outputs data (a correction constant number specified for each heating resistor) for the address; 103, an EPROM which receives the data from the EPROM 102 and 6-bit signals C (cyan=blue), M (magenta=red), and Y (yellow) as an address and outputs data (6-bit resistance-corrected C, M, Y signals). Its operation comprises the steps of measuring the resistances of respective heating resistors in advance, grouping the heating resistors by the resistance, causing the EPROM 102 to store data concerning which heating resistor selects which correction constant as a number table by the counter 101, and converting the magnitude of the C, M, Y signals at the EPROM 103 that references to each group number of a correction constant. More specifically, an output signal selected from 0 to 63 levels is generated from an input signal indicating any of 0 to 63 levels in accordance with an error in the resistance of a heating resistor. For example, a 38-level signal corresponding to a 100th heating resistor is recorded after being corrected to a 40-level or 35-level signal. However, such correction is not effective in eliminating density nonuniformity at a high-density side, thereby not allowing high-quality recording comparable to photographs to be achieved. To overcome the problem of density nonuniformity, the measure of converting the magnitude of a gradation level signal is taken in the above example. However, if a gradation level signal to be applied to a heating resistor is, e.g., level 63 and if it is the upper limit in the gradation level scale, then, the corrected level is level 70, such corrected level cannot exhibit its proper value in the recording, and the recording is effected at level 63 instead. As a result, the correction accuracy is impaired, and this imposes the problem of deteriorating the image quality. What has been achieved is only "compressed recording" such as 50-level recording out of 64-level recording (see FIG. 17).
By the way, in such a multi-gradation recording apparatus, if an input signal applied to the thermal head has n gradation, then the maximum drive count to be applied to the thermal head is made coincide therewith. And even though the thermal head is replaced, such maximum drive count has been fixed to n. Further, each gradation level of any of the recording colors Y, M, C, BK has been subjected to the same correction as described above.
The conventional multi-gradation recording apparatus is disadvantageous as follow.
First, the conventional multi-gradation recording system could not correct density nonuniformity at a high-density portion. For example, even if an input signal has 64 gradation levels, the recorded images are compressed to 50 gradations. Therefore, high-quality recording, such as photograph, could not be achieved.
Second, in the conventional multi-gradation recording apparatus, the maximum drive count applied to the thermal head is maintained constant at all times (e.g., a fixed 280-level drive). If the resistance of the heating resistors of the thermal head has been improved and, as a result, the head has been replaced (the maximum output signal of the correction means is converted to level 260), then 20 unrecorded drive signals (280-260=20) are applied, hence producing an idle time.
Third, in the conventional multi-gradation recording apparatus, the density nonuniformity correction means receives no signal corresponding to a color signal. Thus, recording color-based density nonuniformity cannot be corrected completely and thus high-quality recording comparable to photographs cannot be implemented. Specifically, as shown in FIGS. 3A to 3D, a relation between the energy to be applied to the thermal head (the number of conducting pulses) per color and the recording density cannot be expressed as a linear function since the low and high density portions have gradual inclinations. To overcome this circumstances, a recording density is segmented uniformly. For example, as shown in FIG. 3A, for signal Y, the number of pulses are defined in advance in such a manner that: level 1 is set to 10 pulses; level 2, to 16 pulses; . . . ; and level 64, to 250 pulses (in 64-level recording) so that densities of respective gradation levels can be reproduced properly. Such correspondence is prepared every recording color Y, M, C, or Bk if necessary. That is, the number of pulses (energy) for a gradation level differs from one color to another, and if density nonuniformity is corrected independently of recording colors as in the case of the conventional example, then some colors remain uncorrected as shown in FIG. 4 (colors Y, M, Bk in FIG. 4), thereby impairing the image quality in systems in which colors are recorded while overlapping three or four colors one upon the other. In short, the conventional example is only suitable for use in monochrome recording and thus it is far from being qualified as a high-quality recording apparatus.